The present invention relates to a deflection circuit arrangement for controlling the scanning of an electron beam of a cathode ray tube.
A conventional deflection circuit arrangement will be described below with reference to FIG. 6 and FIGS. 7A through 7C. FIG. 6 is a constitution diagram showing a diode modulator type horizontal deflection circuit capable of correcting the side pincushion distortion of a rectilinear raster and a high-voltage generating circuit. FIG. 7A shows a pattern composed of three horizontal bars reproduced on a screen. FIG. 7B shows a horizontal deflection current waveform at the time of reproducing the pattern of FIG. 7A. In FIG. 6, the reference numerals respectively stand for: 2: a horizontal deflection coil, 3: a horizontal deflection output transistor, 4: a first resonant capacitor, 5: a second resonant capacitor, 6: a first damper diode, 7: a second damper diode, 9: a flyback transformer, 10: a capacitor, 11,12: resistors, 14: a diode modulation coil, 15: a horizontal size control circuit.
In a wide-angle deflection cathode ray tube, deflection currents of ideal saw-tooth waveform through the deflection coil distort the reproduced picture on its screen such that the reproduced picture is stretched at the edges of the screen, and the saw-tooth waveform of the deflection current needs to be distorted slightly toward the shape of the letter S. This current waveform distortion is obtained by connecting a capacitor in series with the deflection coil. A capacitor 8 in FIG. 6, which is inserted for this purpose, is commonly called a "S-correction capacitor", this nomenclature being derived from the shape of the above-mentioned waveform.
In such a case as displaying a white raster, a beam current is automatically limited to a certain value not to flow excessively. As shown in FIG. 6, the capacitor 10 is connected in series with the flyback transformer 9 to detect a current flowing into the flyback transformer and the detected current controls the voltages for controlling display contrast and brightness in the video preamplifier IC. This circuit is called an ABL (an Average Brightness Limiter) and is shown as ABL 40 in FIG. 6. The ABL detection signal of ABL 40 in FIG. 6 is used for this purpose.
A first resonant circuit is composed of the first resonant capacitor 4, the horizontal deflection coil 2, and a primary winding of the flyback transformer 9. A second resonant circuit is composed of the second resonant capacitor 5 and the diode modulation coil 14. A horizontal drive signal 50 is applied to a base of the horizontal deflection output transistor 3, thereby flowing the deflection current of saw-tooth waveform of a horizontal deflection period through the horizontal deflection coil 2. At the same time, flyback pulses appearing at a collector of the horizontal output transistor 3 are raised by the flyback transformer 9 to provide an anode voltage.
As is shown in FIG. 7A, if the pattern comprising top and bottom black horizontal bars and a center white horizontal one is reproduced, the anode voltage drops for a portion of signals corresponding to white signals as shown in FIG. 7C. This spreads a portion of the raster associated with white signals horizontally. To correct this distortion, variations in the anode voltage is detected by a combination of resistors 11 and 12, and the obtained anode voltage detection signal is fed to the horizontal size control circuit to suppress variations in horizontal size of a white area of the raster. Such a device has been disclosed in Japanese Patent Application Laid-Open No. Sho 63-279672 and Japanese Patent Application Laid-Open No. Hei 8-9187.
For reproduction of an image pattern containing a white peak as shown in FIG. 7A, the beam current increases considerably at a part of the white peak. The energy corresponding to this increased beam current is compensated by an increase in the current through the primary winding of the flyback transformer 9 from +B power supply. In addition to this current, the S-correction capacitor 8 supplies a current through the horizontal deflection coil 2. The current through the horizontal deflection coil 2 has a combined waveform of the white-peak-related variation component and the saw-tooth wave of horizontal deflection period as shown in FIG. 7B. Otherwise straight vertical lines appear distorted in a right-left asymmetric fashion on the screen as shown in FIG. 7A.